Si.sub.3 N.sub.4 materials and parts are normally produced by mixing sinterable Si.sub.3 N.sub.4 starting powder characterized by high fineness and purity with sintering additives, such as MgO, Y.sub.2 O.sub.3, Al.sub.2 O.sub.3 and/or ZrO.sub.2, in concentrations of&lt;15% by weight, molding the resulting mixture and subsequently densifying the molding by a heat treatment. The heat treatment may be carried out under atmospheric nitrogen pressure by sintering or axial hot pressing or under elevated gas pressure in N.sub.2 or a mixture of N.sub.2 and an inert gas by gas pressure sintering or hot isostatic pressing. The reaction of the sintering additives with the oxygen typically present in silicon nitride starting powders, in which it may be assumed to be present as SiO.sub.2 or Si.sub.2 N.sub.2 O, produces a liquid phase so that sintering mechanisms, such as rearrangements induced by capillary forces and solution-reprecipitation processes, can occur. These processes are promoted by oxidic impurities, such as alkali metals, alkaline earth metals, iron and other metal oxides, because they contribute towards formation of the liquid phase. After densification, the liquid phase remains in the material, generally as a glass-like secondary phase at grain boundaries and triple points, and adversely affects the properties of the material, above all at relatively high temperatures.
In addition to this adverse effect on the mechanical properties by the grain boundary phase, other impurities, such as elemental Si, metals or silicides, which melt at temperatures below the sintering temperature, can impair the mechanical properties because the spherical droplets formed during melting are no longer dissolved by the liquid phase. These droplets remain in the material as foreign particles after the solidification of the liquid phase and, because of their different thermal expansion and elastic properties by comparison with the matrix material, lead to stress concentrations which often act as failure-initiating defects (Evans, A. G.: Non-Destructive Failure Prediction in Ceramics, in: F. L. Riley (ed.) "Progress in Nitrogen Ceramics", Martinus Nijhoff Publ., Boston, 1983, pages 595 to 625).
Accordingly, since the strength of a material is reduced by such inclusions, a quality criterion for Si.sub.3 N.sub.4 powders is that their content both of metallic impurities and of free silicon should be as low as possible.
In addition to these particulate inclusions, large pores, voids or internal cracks, of which the causes can generally be attributed to inhomogeneities in the starting powder, granule defects or stresses in the moldings, act as failure-initiating defects and have a strength-reducing effect accompanied by an increase in the spread of the properties. In many cases, stress-induced cracks are formed on heating to the sintering temperature, being promoted by thermal gradients in the specimens.